Negotiations are inherently dyadic. Negotiators’ individual-level characteristics may not only make them perform better or worse in general, but also may make them particularly well- or poorly-suited to negotiate with a particular counterpart. The present research estimates the extent to which performance in a distributive negotiation is affected by (a) the negotiators’ individual-level characteristics and (b) dyadic interaction effects that are defined by the unique pairings between the negotiators and their counterparts. Because negotiators cannot interact multiple times without carryover effects, we estimated the relative importance of these factors with a new methodology that used twin siblings as stand-ins for each other. Participants engaged in a series of 1-on-1 negotiations with counterparts while, elsewhere, their cotwins engaged in the same series of 1-on-1 negotiations with the cotwins of those counterparts. In these data, dyadic interaction effects explained more variation in negotiation economic outcomes than did individual differences, whereas individual differences explain more than twice as much of the variation in subjective negotiation outcomes than did dyadic interaction effects. These results suggest dyadic interaction effects represent an understudied area for future research, particularly with regard to the economic outcomes of negotiations.